The present invention relates to a variable characteristics amplifier circuit, and more particularly to a variable characteristics amplifier circuit used in an inter-music interval detecting device for a tape deck or the like.
Recently, tape decks have been developed in which blank intervals between music intervals recorded on a tape are detected for the purpose of controlling special reproducing operations such as repeating selections, scanning, skipping blank portions or the like in the playback mode, and reviewing, jumping, selecting music, or the like in the fast forward or rewinding mode. In the fast forward and rewinding modes, the transport speed of the tape is generally higher than in the playback mode, and the frequency band of the read-out audio signal is higher than in the playback mode by an amount corresponding to the ratio of tape speeds in the two modes. Also, the output level of the signal from the reproducing head is at a generally higher level than in the playback mode. Therefore, in the inter-music interval detecting device, it is necessary to employ different gain levels and different cut-off frequencies for the output signal from the reproducing head in the two modes in order to prevent errors due to noise in detecting the intermusic intervals. Accordingly, there is provided a variable characteristics amplifier circuit to vary the gain and frequency characteristics of the reproduced signal in accordance with the operating mode.
FIG. 1 shows an example of a conventional variable characteristics amplifier circuit used for an inter-music interval detecting device in a tape deck. In this drawing, left-channel (L) and right channel (R) audio signals are reproduced from a tape by a reproducing head (not shown) and then applied to succeeding-stage amplifiers (not shown) through preceding-stage amplifiers 1.sub.L and 1.sub.R , coupling capacitors 2.sub.L and 1.sub.R , and buffer amplifiers 3.sub.L and 3R, respectively. The respective output terminals of the preceding-stage amplifiers 1.sub.L and 1.sub.R are connected to a variable-characteristics amplifier circuit 26 through resistors 5.sub.L and 5.sub.R, respectively. Each of the resistors 5.sub.L and 5.sub.R has a comparatively large resistance value so as to prevent loss of separation between the L and R signals. The operation of the tape deck is controlled by a microcomputer 4 having output ports O.sub.1 to O.sub.4. The level of the signal at the output port O.sub.4 is low for the playback mode and high for the fast forward and rewinding modes.
An input section of the variable characteristics amplifier circuit 26 is constituted by an emitter-follower circuit 25 composed of a resistor 6 and a transistor 7. The emitter of the transistor 7, constituting an output terminal of the emitter follower circuit 25, is connected to an anode of a diode 10 through a capacitor 8 and a resistor 9. A voltage V.sub.DD is supplied to the anode of the diode 10 through resistors 11 and 12 connected in series. The line connecting the resistors 11 and 12 is grounded through the collector-emitter circuit of a transistor 13. The base of the transistor 13 is connected to the output port O.sub.4 of the microcomputer 4 through a resistor 14. A capacitor 15 is connected in parallel with the diode 10, the cathode of which is grounded through a resistor 16. The cathode of the diode 10 is connected also to an inverting input terminal of an operational amplifier 18 through a capacitor 17. A resistor 19 and a capacitor 20 are connected in parallel with each other between the input and output terminals of the operational amplifier 18 so as to constitute an inverting amplifier circuit. A Schmitt-trigger amplifier 21 is connected to an output terminal of the operational amplifier 18, that is, an output terminal of the variable characteristics amplifier circuit 26, and an output terminal of the Schmitt-trigger amplifier 21 is connected to an input port I.sub.1 of the microcomputer 4 through an inverter 24 constitutedd by a transistor 22 and a resistor 23.
In this arrangement, the respective audio signals produced by the preceding-stage amplifiers 1.sub.L and 1.sub.R are summed through the resistors 5.sub.L and 5.sub.R, and the summed signal is applied in the form of a monoaural signal from the emitter of the transistor 7 of the emitter follower 25 to the anode of the diode 10 through the capacitor 8 and the resistor 9.
Assuming now that the tape deck is in the playback mode, the level of the signal at the output port O.sub.4 of the microcomputer 4 is low, and thus the transistor 13 is off. Thus, the voltage V.sub.DD is applied to the series circuit constituted by the resistors 11 and 12, the diode 10, and the resistor 16, and hence current flows in the series circuit, causing the diode 10 to be in the on state.
Thus, an output audio signal of the emitter follower circuit 25 is applied to the operational amplifier 18 through the capacitor 8, the resistor 9, the diode 10, and the capacitor 17 without passing through the capacitor 15. The operational amplifier 18 inverts and amplifies the input signal applied thereto. If the sum (R.sub.11 +R.sub.12) of the resistance values R.sub.11 and R.sub.12 of the resistors 11 and 12 and the resistance value R.sub.16 of the resistor 16 are sufficiently larger than the resistance value R.sub.9 of the resistor 9, and if the output impedance of the emitter follower circuit 25 is sufficiently smaller than the resistance value R.sub.9 of the resistor 9, the gain G.sub.V in the intermediate frequency band of the operational amplifier 18 will be G.sub.V .congruent.20 log R.sub.19 /R.sub.9, where R.sub.19 represents the resistance value of the resistor 19. A frequency f.sub.H which is 3 dB below the maximum value in the high frequency band is expressed by f.sub.H .congruent.1/2.pi.C.sub.20 R.sub.19, where C.sub.20 represents the capacitance value of the capacitor 20. The cut-off frequency f.sub.L in the low frequency band can be expressed by f.sub.L .congruent.1/2.pi.C.sub.17 R.sub.9 if the capacitance value C.sub.17 of the capacitor 17 is sufficiently smaller than the capacitance value C.sub.8 of the capacitor 8.
On the other hand, when the tape deck is in the fast forward or rewinding mode, the level of the signal at the output port O.sub.4 of the microcomputer 4 is high so that the transistor 13 is turned on. Upon turning the transistor 13 on, the line between the resistors 11 and 12 substantially becomes ground potential so that the diode 10 is turned off, that is, no bias voltage is applied to the diode 10. Accordingly, the output audio signal from the emitter follower circuit 25 is applied to the operational amplifier 18 through the capacitor 8, the resistor 9, and the capacitors 15 and 17.
In this case, if the operational amplifier 18 were ideal, the gain G.sub.V in the intermediate frequency band would be G.sub.V .congruent.20 log R.sub.19 /R.sub.9, as in the playback mode. However, the operational amplifier 18 cannot in actuality be regarded as ideal, and further if the resistance value R.sub.12 of the resistor 12 is small, the input signal level is attenuated and therefore the gain imparted by the operational amplifier 18 is reduced. In this case, the cut-off frequency f.sub.L in the low frequency band becomes f.sub.L .congruent.1/2.pi.C.sub.15 (R.sub.9 //R.sub.12), where C.sub.15 represents the capacitance value of the capacitor 15 (C.sub.15 &lt;&lt;C.sub.17) and R.sub.9 //R.sub.12 the resistance of the parallel combination of the resistors R.sub.9 and R.sub.12 (=R.sub.9 R.sub.12 /(R.sub.9 +R.sub.12)). Therefore, in the fast forward mode as well as in the rewinding mode, the gain in the intermediate frequency band is reduced and the cut-off frequency in the low frequency band is made high in comparison with the playback mode.
The output signal of the operational amplifier 18 is compared with a predetermined level in the Schmitt-trigger trigger amplifier 21. When the output signal of the operational amplifier 18 is not smaller than the predetermined level, the output level of the Schmitt-trigger amplifier 21 is high, while when it is below the predetermined level, the output level of the Schmitt-trigger amplifier 21 is inverted by the inverter 24 and applied to the input port I.sub.1 of the microcomputer 4. When the level at the imput port I.sub.1 is low, the microcomputer 4 determines that no reproduced signal is then present, that is, that an inter-music interval is indicated.
However, in such a conventional variable characteristics amplifier circuit, there are disadvantages in that it is difficult to determine the circuit constants because the on resistance of the diode 10 cannot be actually disregarded. Moreover, variations in the gain of the operational amplifier 18 result in variations in the gain in the intermediate frequency band because the operational amplifier 18 does not act as an ideal amplifier when the diode 10 is turned off.